In electrical indicators, often classified as analog devices such as volt meters, ammeters, and other dial and pointer-type instruments, it has been necessary to address the problems associated with returning the pointer to zero when no power is applied to the meter. Over the course of many years attempts have been made to provide this function by a variety of means, including electrical, mechanical, magnetic, or combinations thereof. In some types of meter movements, it is relatively easy to provide means to return the pointer to zero. However, in air core or vector movement meters where springs are not used, when the electrical power is turned off, the pointer does not automatically return to zero, but it will normally remain in the position it was when the power was turned off. On the other hand, the pointer may come to rest at some arbitrary position unrelated to the reading at the time the power is disconnected. In some instances, the fact that the meter does not return to zero is a cosmetic problem and more a matter of choice than a matter of necessity. However, in some applications there could be a direct relationship between safety and the reading of a meter with no power applied. For example, in aircraft, if there is a loss of electrical power to an instrument, the fact that it does not return to zero can result in the operator's perceiving a false reading and thereby reacting improperly to a situation where he does not have accurate information.
Air core meter movements have replaced the D'Arsonval movements, which were the standard for analog meters for many years, in many automotive uses. This is because the air core movement is more rugged than the D'Arsonval movement, the air core movement has no springs, and the torque applied to it is high enough so that the bearing arrangement may be formed as a rugged throughshaft system as opposed to employing jeweled bearings. An air core instrument employing coils which generate orthogonal magnetic fields for purposes of deflecting the movement rotor is shown in U.S. Pat. No. 3,168,689. Another aspect of air core meters is shown in U.S. Pat. No. 3,460,038. These patents describe the structure and operation of a basic air core meter, the type of meter which is the subject of the present invention.
Examples of patents which disclose particular means for providing a return-to-zero function, and patents which disclose the use of auxiliary permanent magnets within the meter movement for some purpose are listed below.
U.S. Pat. No. 3,777,265 employs magnets for the zero or restoring force, and an external electrical current is employed to provide the field for deflecting the meter pointer.
U.S. Pat. No. 3,995,214 employs magnets for biasing and compensating only for purposes of improving the linearity of the meter response to input signals.
Another example of a patent which employs a stationary permanent magnet to provide return torque is U.S. Pat. No. 4,090,131. In this patent, the magnetic field of the permanent magnet interacts with the rotor flux to provide a return torque which varies linearly with the rotor angle.
Other patents which disclose the use of permanent magnets for zeroing the meter movement are U.S. Pat. Nos. 2,668,945 and 3,094,659.